This application is based upon Japanese Patent Application Nos. Hei. 11-171650 filed on Jun. 17, 1999, Hei. 11-171651 filed on Jun. 17, 1999, and Hei. 11-244306 filed on Aug. 31, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to angular position detection apparatuses, and in particular to an angular position detection apparatuses containing a permanent magnet which is magnetized so that lines of magnetic force are parallel with each other.
2. Related Art
In an angular position detection apparatus, which detects for example an opening degree of a throttle valve (an opening degree of a throttle) of an internal combustion engine, as shown in FIG. 22, a stator core 12 is concentrically arranged at an inner side of a cylindrical rotor core 11, which integrally rotates with throttle valve (not-shown). Two permanent magnets 13 having circular arc are fixed to the inner side of the rotor core 11 so as to confront each other with sandwiching the stator core 12. Each of the permanent magnets 13 is radially magnetized so that all of lines of magnetic force in the permanent magnets 13 are set to a radial direction. Here, in FIG. 22, directions of lines of magnetic forces in each of parts are illustrated by arrows (xe2x86x92). Incidentally, a magnetic flux detection gap portion 14 having a constant width is formed at a center portion of the stator core 12 so as to open along a diametral direction. A magnetic detection element 15 such as a Hall IC is provided in the magnetic flux detection gap portion 14.
According to this structure, as shown in FIG. 24, since a magnetic flux density going through the magnetic flux detection gap portion 14 of the stator 12 (magnetic flux density crossing the magnetic detection element 15) changes in proportion to the angular position of the rotor core 11, and since output of the magnetic detection element 15 changes in proportion to the magnetic flux density, the angular position of the rotor core 11 (angular position of the throttle valve) is detected based on the output of the magnetic detection element 15.
In the above-described structure, the output of the magnetic detection element 15 for detecting the angular position of the rotor core 11 changes in proportion to the magnetic flux density in the magnetic flux detection gap portion 14. Therefore, if the magnetic flux in the magnetic flux detection gap portion 14 linearly changes in proportion to the angular position of the rotor core 11, since an output characteristic of the magnetic detection element 15 regarding the angular position becomes linear, a detection characteristic of the angular position is improved.
However, according to the conventional structure discussed above, since the permanent magnets 13 are radially magnetized, it is impossible to secure a wide range in which magnetic flux in the magnetic flux detection gap portion 14 linearly changes in proportion to angular position of rotor core 11. Therefore, since linear output with respect to angular position can be obtained over only a relatively narrow range, detection accuracy of the angular position decreases. For example, as shown in FIG. 24, a range, in which the magnetic flux in the magnetic flux detection gap portion 14 linearly changes in proportion to the angular position of the rotor core 11, is at most approximately 80xc2x0 (degrees). Therefore, when the angular position exceeds 80xc2x0 (degrees), it is impossible to obtain linear output with respect to angular position, and detection accuracy of the angular position decreases.
Furthermore, since the permanent magnets 13 are arranged so as to confront with an outer surface of the stator core 12, an outer size of the rotor core 11 for fixing the permanent magnets 13 and therefore an outer size of the angular position detection apparatus becomes bulky.
Moreover, in order to uniformly radially magnetize the permanent magnets 13, a density of the inner side of the permanent magnet 13 needs to be dense and a density of the outer side thereof needs to be coarse. Therefore, strength of the permanent magnet 13 is likely to decrease due to a difference in density.
This invention has been conceived in view of the background thus far described and its first object is to provide an angular position detection apparatus in which a linearity of an output characteristic with respect to an angular position can be improved so as to improve a detection accuracy of the angular position.
Its second object is to improve strength of a permanent magnet.
Its third object is to downsize an outer size of the angular position detection apparatus.
According to one aspect of the present invention, a permanent magnet is fixed to a side portion of a rotor core, so that the permanent magnet does not confront with an outer surface of a stator core. Thus, an outer size of the rotor core and therefore an outer size of the angular position detection apparatus can be downsized.
In this case, it can be thought of as an axial gap in which a gap toward an axial direction (shaft direction) between the permanent magnet and the stator core exists, so that magnetic flux goes through the gap along the axial direction. However, one needs to form the air gap toward the axial direction between the permanent magnet and the stator core uniform and small, in order to improve linearity of the output characteristic of a magnetic detection element. In this case, it needs to precisely control a ratio of flatness and a ratio of the parallelism of confronted surfaces of the permanent magnet (rotor core) and the stator core, and therefore it is hard to manufacture.
According to this structure, although the permanent magnet is fixed to the side portion of the rotor core, the angular position detection apparatus is not formed as an axial gap type, but is formed as the radial gap type in which a cylindrical portion formed at the outer portion of the rotor core is closely confronted with the outer surface of the stator core, so that the magnetic flux goes through the air gap between the cylindrical portion and the stator core along the radial direction. As a result, when the axis between the rotor core and the stator core can be accurately controlled to be coaxial, the dimensional accuracy of the air gap can be also controlled. Furthermore, since the permanent magnet is fixed to the side portion of the rotor core, the permanent magnet can be formed in a flat plate shape, and therefore it can facilitate manufacture of the permanent magnet.
Moreover, in this structure, the permanent magnet is magnetized so that lines of the magnetic force in the permanent magnet are parallel with respect to each other (parallel magnetization). Thus, it can expand the range in which the magnetic flux density in the magnetic flux detection gap portion of the stator core linearly changes in proportion to rotation of the permanent magnet, compared to the radial magnetization in the related art. As a result, it can obtain linear output with respect to angular position over a wider range compared to the related art, and it can improve the detection accuracy of angular position. Furthermore, parallel magnetization can make the density of the permanent magnet uniform, so that the strength of the permanent magnet can be increased.
Incidentally, for example, in an electric throttle system, a throttle valve is rotated by an actuator such as a motor via a reduction mechanism, and angular position of the actuator is detected by an angular position apparatus to detect the angular position of the throttle valve (throttle opening degree). However, in this structure, since the throttle opening degree is calculated by dividing detected notation of the actuator by the reduction ratio of the reduction mechanism, detection error of the throttle opening becomes greater due to variation in the reduction ratio or backlash (backlash) between gears.
To solve the disadvantage, there is an electrical throttle system in which an angular position detection apparatus is externally provided on a extended line of a rotational shaft of the throttle valve, and a tip of the rotation shaft of the throttle valve is connected to a rotor in the angular position detection apparatus, so that the angular position of the throttle valve (throttle opening degree) is directly detected. However, according to this structure, since the angular position detection apparatus is provided at outer side of the cover, which covers the reduction mechanism, entire system may be bulky. Moreover, since the tip of the rotation shaft of the throttle valve is connected to the rotor in the angular position detection apparatus, a rotation resistance of a bearing, which supports the rotor in the angular position 5 detection apparatus, acts to the rotation shaft of the throttle valve. Thus, the rotation resistance of the throttle valve becomes large, and therefore the load of the actuator becomes large.
Therefore, according to another aspect of the present invention, a permanent magnet may be fixed to a rotation shaft of the rotating body such as a throttle valve, a magnetic detection element for detecting magnetic flux of the permanent magnet may be fixed at inner side of the cover, which covers a reduction mechanism. Thus, since the angular position of the rotating body is directly detected, the angular position of the rotating body can be accurately detected. In addition, since an angular position detection apparatus (permanent magnet and magnetic detection element) can be contained in an inside space of the cover, which covers the reduction mechanism), the entire system can be downsized compared to the related art. Since the rotor (permanent magnet) of the angular position detection apparatus is supported by the rotation shaft of the rotation body, it does not need to support the rotor (permanent magnet) of the angular position detection apparatus by a bearing. As a result, it can reduce a rotation resistance of the rotation shaft of the rotation body, which rotates the rotor of the angular position detection apparatus, and it can reduce a load of an actuator.
According to still another aspect of the present invention, a permanent magnet to be fixed to a rotor core is magnetized so that lines of magnetic force in the permanent magnetic are set to parallel (parallel magnetization). When the permanent magnet is magnetized in parallel, it can expand a range, in which the magnetic flux density in the magnetic flux detection gap portion of the stator core linearly changes in proportion to a rotation of the permanent magnet, compared to the radial magnetization as the related art. As a result, it can obtain the linear output with respect to the angular position in wider range compared to the related art, and it can improve the detection accuracy of the angular position. Furthermore, the parallel magnetization can make the density of the permanent magnet uniform, so that the strength of the permanent magnet can be increased.